There are many types of polyethylene made and sold today. One type in particular is made by various suppliers and sold in large quantities. This polyethylene is called high pressure free radical polyethylene (usually called LDPE) and is usually made using a tubular reactor or an autoclave reactor or sometimes a combination. Sometimes polymer users blend LDPE with other polymers such as linear low density polyethylene (LLDPE) to try to modify properties such as flowability or processability.
We have now discovered new LDPE polymers which have improved extrusion coating properties and can have improved shrinkage in combination with favorable stiffness, tensile strength, melt strength and optics, while maintaining other performance attributes.
For example, S. Goto et al; Journal of Applied Polymer Science: Applied Polymer Symposium, 36, 21-40, 1981 (Ref. No. 1) has the following general discussion regarding reaction kinetics. Low density polyethylene resins with higher densities (≧926 kg/m3) are produced at reduced polymerization temperature in order to reduce the short chain branching frequency and consequently to increase product density. Both the reaction rate of the short chain branching (also known as backbiting) as well as the long chain branching (also known as transfer with polymer) reaction step are very temperature dependent.
The table below shows the kinetic data on the involved reaction steps (Ref. No. 1). The temperature dependence is given by the activation energy. The higher the activation energy the more a certain reaction step will be promoted by higher temperature or reduced by lower temperatures.
Rate Constants of Elementary Reaction Rates (Ref. No. 1)FrequencyActivation energy,Activation volume,Reaction stepfactorcal/molecm3/molePropagation5.63E+1110,520−19.7LCB1.75E+1214,0804.4SCB5.63E+1213,030−23.5
For polymer properties the ratio between the rate of a certain reaction step and the propagation rate is of importance.
The property temperature dependence is expressed by the Δ Activation energy, so for:
SCB frequency in product: Δ Activation energy=13.03−10.52=2.51 kcal/mole
LCB frequency in product: Δ Activation energy=14.08−10.52=3.57 kcal/mole
Above data imply that the LCB frequency decreases faster than the SCB frequency with decreasing temperature. Further the lower maximum polymerization temperatures needed to lower the SCB frequency will also lower the polymer concentration (/profile) in the reactor. Since the LCB reaction rate also depends on polymer concentration, the LCB frequency will be lowered furthermore when increasing polymer density. This means that the LCB frequency is both lowered by the lower polymerization temperature as well as by the lower polymer concentration in the reactor when increasing density of LDPE.
The molecular weight distribution of polyethylene is heavily affected by LCB frequency. High LCB frequency leads to broad MWD resins, while low LCB frequency leads to narrow MWD resins. This means that it becomes increasingly difficult and at some point impossible to produce broad MWD polyethylene resins when increasing polymer density. Broad MWD polyethylene is needed for a variety of extrusion applications, specifically to control the rheology in the molten state. An example is the need for low neck-in during extrusion coating.